In motor control the highest performance can be achieved when accurate motor rotor angular position and velocity information are available. Various technologies have been used to measure the shaft angle like hall-effect angle sensing with a tachometer for velocity sensing. Another common solution has been the use of a resolver, paired with a resolver to digital converter.
A rotating electric resolver is an electromagnetic device which is used to accurately indicate the angular position of the rotor with reference to some fixed frame of reference. A typical embodiment of such a resolver comprises an input winding and a pair of output windings. The output windings are typically identical to each other but are wound with a 90 degree phase displacement relative to each other.
These technologies, while effective, are costly and large in both physical size and weight.